Circuit Switched (CS) FallBack (CSFB) and Single Radio Voice Call Continuity (SRVCC) are features that were introduced in Third Generation Partnership Project (3GPP) Release 8.
The CSFB in Evolved Packet System (EPS) enables the provisioning of voice and other domain services by reuse of CS infrastructure when a UE is served by Evolved-Universal Terrestrial Radio Access Network (E-UTRAN), i.e. using a CS network to provide voice services alongside of an Long Term Evolution (LTE) network, i.e. a Packet Switched (PS) network. A CSFB enabled UE, connected to E-UTRAN may use GERAN or UTRAN to connect to the CS domain. This function is only available in case E-UTRAN coverage is overlapped by either GERAN coverage or UTRAN coverage. GERAN is short for GSM EDGE Radio Access Network, GSM is short for Global System for Mobile communications, EDGE is short for Enhanced Data rates for Global Evolution and UTRAN is short for Universal Terrestrial Radio Access Network. In other words, CSFB is a mechanism where the LTE network moves the UE to a legacy network which supports voice call. CSFB is used if the Internet protocol Multimedia Subsystem (IMS) network is not available or if the UE is not able to do Voice over LTE (VoLTE) for some reason (for example registration to the IMS network failed). The term VoLTE is used to describe voice and Short Message Service (SMS) in LTE. The term legacy network is used to refer to any older network, i.e. a network which is older than the current network, a network based on older network standards than the current network standard.
SRVCC is defined by the 3GPP as “Voice call continuity between IMS over PS access and CS access for calls that are anchored in IMS when the UE is capable of transmitting/receiving on only one of those access networks at a given time”. SRVCC is a process where an active voice call in LTE is handed over to the CS domain of the legacy network. This may be useful when the UE is in an LTE coverage area and have an active voice call over IMS and the UE moves out of the LTE area into a legacy only network coverage. Instead of the call getting dropped, the same will be handed over to the CS legacy network if SRVCC is supported.
When the UE is not reachable by e.g. a PS network, the UE may be in suspend state or power saving state, or the serving mobility management node has cleared a Paging Proceed Flag (PPF) flag for the UE. When the PPF is cleared its value may be set to “false” or to the value 0 or similar which indicates that the flag is cleared. When the UE gets into suspend state, the mobility management node (e.g. a Mobility Management Entity (MME) and/or a Serving General packet radio service Support Node (SGSN)) will inform the gateway node (e.g. Serving GateWay (SGW) and/or Packet data network GateWay (PGW)) about the suspension. When the UE is in suspended state, UE does not send any user data nor does the UE send any signaling information. When the UE is in suspend state, the UE may be unavailable in the PS domain for PS services due to that the UE is in service in the CS domain (for a UE or network that cannot do CS and PS at the same time). The power saving state was introduced by 3GPP in Release 12. The UE may enter power saving state on its own initiative, and the UE may wake up at the next periodic Tracking Area Update (TAU) message or the next Routing Area Update (RAU) message, or if the UE has any uplink data or signaling to send. Information indicating that the UE is entering power saving state is not reported to gateway nodes such as the SGW or PGW. When the UE is unreachable (i.e. not reachable) it does not hear any paging that is sent to it by the MME/SGSN, and will therefore not answer. When the UE gets into a power saving state, the MME/SGSN is the only network node which holds information about the UE being in the power saving state. According to the existing requirement, if the MME/SGSN has received some network signaling this may trigger further signaling towards the unreachable UE. Such further signaling from the MME/SGSN towards the unreachable UE may be deemed failed.